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The effective index profiles in a vertical coupler of SOI and a-Si. The cross-over points indicate at which position optical impedance matching between SOI and a-Si waveguides is achieved. The horizontal dashed and dotted lines represent untapered a-Si and SOI waveguides, respectively.

(a) We estimate waveguide transmission loss by integrating sidewall roughness scattering and bulk absorption loss along taper structures; (b) corresponding mathematical formula, where li is the unit length at wi, ΓiaSi is the confinement factor for a-Si at wi, αiSOI and αiaSi are the sidewall scattering loss coefficients for SOI and a-Si at wi, αbulkaSi is the bulk absorption coefficient for a-Si. While αi and Γi is a function of wi, αbulk is constant for a-Si. Here, we assume SOI silicon does not have bulk absorption at 1550 nm, αbulkSOI = 0.

Tables (1)

Table 1. Measured total coupling loss, α(C), and simulated transmission loss, α(T), for the 9 different vertical couplers, e.g. α30 is the loss coefficient for 30 μm long coupler. The loss is given in dB.